Roofing - Wish to put 90lb rolled over existing torch-down roof.

I have two 24w x 16deep 2/12pitch roof segments done with a layer of appx 15yr old modified bitumen/torch-down. One each on front and back of my 24' wide home. Between the front and back "low slopes" is a 10/12 pitch main roof thats shingled.
The torchdown is not leaking, but it about end of life and showing small cracks all over.
I'm doing the high slope shingle job anyways, So I'd like to put down a layer of 90lb rolled roofing over the low slope while i'm up there. The decking is solid, and both slopes are single/only layer.
As far as putting rolled over torchdown is concerned, should I brush on a coat of roofing cement or coating between the layers (in addition to the nailing and lap cement of course)
How should I deal with the low slope?
Thanks !
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